This application claims priority from German Application 102008015851.8, filed Mar. 27, 2008.
The present invention relates to a fluid-driven ultrasonic welding tool, wherein the ultrasonic welding tool is connected by way of a piston rod to a working piston which is guided in a housing of the drive, wherein the working piston sub-divides the drive housing into a primary chamber and a secondary chamber, wherein each of the chambers is able to communicate by way of a valve with a fluidized pressure source.
In this context, the term, “valve” means any device which is able to optionally connect or separate the chamber in question to or from the fluidized pressure source. Clearly, said function of the two valves can also be realized by one single control valve, by means of which it is possible to optionally connect either the primary chamber or secondary chamber to the fluidized pressure source. The term, “fluid” can refer to both gases and liquids.
Fluid-driven ultrasonic welding tools of this kind are known. Usually, the valve connecting to the primary chamber is opened in order to allow the ultrasonic welding tool to move. As a result, the primary chamber communicates with a pressurized air source, and so the pressure in the primary chamber rises suddenly. Since the secondary chamber communicates with ambient pressure at the same time, the working piston moves so that the ultrasonic welding tool is moved.
The ultrasonic welding tool generally has what is known as a sonotrode by means of which ultrasonic fluctuations are transmitted into the material which is to be processed. The ultrasound is produced by a converter acted upon by high-frequency alternating voltage, and that converter may possibly be connected to the sonotrode by means of an amplitude transformer.
In order to move the sonotrode away from the material for processing after the welding has been done, the primary chamber is separated from the pressurized air source and communicates with ambient pressure. As soon as only ambient pressure is present in the primary chamber the pressure in the secondary chamber is greater than in the primary chamber, and so the working piston is set in motion, the primary chamber becomes smaller in size, and the secondary chamber becomes larger in size.
This mode of operation has the drawback that only the desired working pressure, i.e. the pressure which the sonotrode exerts upon the material to be processed, can be set. There is virtually no way of influencing the actual course of movement of the sonotrode.
Furthermore, the secondary chamber is sometimes subject to a counter-pressure, in order to allow various different tools to be operated. However, the nominal pressure in the secondary chamber cannot be adjusted easily. Therefore, the amount of compressed air which is introduced into the secondary chamber at the start of the welding process has to be adjusted accordingly.
Also, the direction of forward movement of the sonotrode is generally in the vertical direction, and so the inherent weight of the sonotrode likewise has an impact upon the working piston. An exchange of the ultrasonic working tool or sonotrode, which can be associated with a change in weight of the tool, therefore means that the counter-pressure in the secondary chamber has to be reset. During the welding process, i.e. whilst the ultrasonic welding tool is moving, the counter-pressure in the secondary chamber must, however, be kept constant.
Since, for some applications, it is advantageous if the lowering speed of the working piston can be set in order to achieve an optimum weld result, the proposal has already been made to drive the ultrasonic welding tool by means of a servo motor. Here, a motor drives a spindle which is connected to the ultrasonic welding tool. The servo motor has the advantage that every pattern of movement of the sonotrode can theoretically be preset by the servo motor. Therefore, often, a position sensor is additionally provided for determining the position of the sonotrode (or of the working piston which is connected to the sonotrode), so that the movement of the sonotrode can be controlled.
However, as a result of external factors, such as, for example, a variation in the thickness of the material to be processed, a change in working pressure can occur, and thus a change in the force to be applied, and this is not detected by the servo motor drive, thereby adversely affecting the weld result.
Even if a force sensor were to be provided as an alternative to, or in addition to, the positions sensor, control by means of a servo motor would find difficulty in counteracting sudden changes of pressure.